Active matrix liquid crystal displays (AMLCD) are formed from an addressing matrix (i.e., an active matrix) and an electro-optically responsive layer that includes multiple liquid crystal cells (i.e., a liquid crystal layer). The addressing matrix includes active devices which are addressed by external addressing lines connected to driver electronics. An active device is associated with one or more local electrodes providing an electrical field to act upon the liquid crystal (LC) layer. Typically a particular active device is associated with a picture element or pixel in the AMLCD. The active matrix separates the addressing of the pixels from an electro-optical response of the LC layer. An electrical field on a pixel set by an active device and suitable switching waveforms provide a desired optical output such as a grayscale shade, etc.
Two types of active devices may be used in AMLCD. One is a three-terminal device such as a Thin Film Transistor (TFT) device, and another is two-terminal device such as a Metal-Insulator-Metal (MIM) device. Each type has their advantages and disadvantages. In general, TFT devices make use of rigid substrates (i.e., glass substrates) and silicon processing to form the active devices. Typically silicon processes are best effected at relatively high temperatures (i.e., greater than 300° C.). Furthermore, since the complete addressing active matrix is formed on one substrate, and the device itself is complex, several aligned lithographic processes may be needed (e.g., more than four processes). This requires the use of a dimensionally stable, high temperature substrate such as glass, rather than a low temperature dimensionally unstable polymer based substrate.
MIM devices rely on non-linear behavior of certain dielectrics (e.g. oxides such as tantalum oxide or Ta2O5), which may be formed at low temperatures (i.e., less than 200° C.). An addressing matrix is formed on both front plane and back plane of a display. Therefore, in general MIM devices are simpler to manufacture than a TFT device, since there are fewer process (i.e., less than four aligned lithographic processes).
It may be advantageous for large area applications to form a display from plastic substrates, rather than thin glass substrates that are typically used. The use of plastic substrates may limit upper process temperatures and limit the number of aligned lithographic processes due to the dimensional instability of a plastic substrate. Therefore, when using plastic substrates, MIM devices may be preferred over TFT devices.
A MIM device may functionally behave like a capacitor having a non-linear current/voltage (I/V) characteristic. In other words, current does not flow up until a threshold voltage is exceeded, after which the MIM device presents relatively low impedance. The threshold voltage is observed in both applied polarities, and often the MIM device is modeled as a capacitor in parallel with a pair of diodes in a “back to back” arrangement.
A single MIM device in series with the addressed liquid crystal pixel has an effect on a charge blocking device, such that once a pixel voltage corresponding to a desired optical output has been achieved, further charge is not passed to or from the pixel and that optical state is held until the pixel is next addressed.
Due to the non-symmetric nature of the interfaces between the insulator and the metal contacts, the forward and reverse threshold voltages may be different. In the single MIM case, this may lead to liquid crystal cell polarization and an effect known as “image sticking”. To overcome the “image sticking” effect, it is well known in the art to use two MIM devices in “anti-series” fashion. The two MIM devices are typically referred to as a “dual-MIM” device. The dual-MIM device arrangement provides an ability to cancel out forward bias effects of one MIM device with reverse bias effects of the other MIM device, and also reduces the capacitive coupling of the aggregate dual-MIM device. One disadvantage is that the maximum current is reduced and the overall threshold may be increased. Furthermore, the additional complexity of the matrix may affect the overall manufacturing yield.
Traditionally MIM devices are formed by deposition, photo-lithography, material conversion and etching in a largely subtractive manner on the surface of the final display substrate. The processes applied are thus limited by the substrate material and the deleterious effect of the protrusion and non-planarity of the resulting structure into the display cell on the liquid crystal electro-optical effect.
Accordingly, the need exists for new and improved systems and methods to fabricate MIM devices or active addressing elements for use in AMLCD.